1. Field of the Invention
The present invention relates to a method for effectively removing harmful and toxic substances of a generally highly metal-corrosive exhaust gas with or without dust discharged from a semiconductor manufacturing process, which exhaust gas particularly contains one or more component gases that are harmful and toxic to human, and flammable or explosive, and that are hydrolyzable, water-soluble or thermally decomposable. The invention further relates to a compact apparatus for removing harmful and toxic substances of such an exhaust gas utilizing the above method, which apparatus can be used along with a semiconductor manufacturing apparatus in a clean room.
2. Discussion of the Background
An exhaust gas discharged from a semiconductor manufacturing process is generally highly metal-corrosive, is flammable and explosive, and contains substances that are harmful and toxic to human beings. It is a conventional practice to dilute such an exhaust gas with a large amount of nitrogen gas to reduce the exhaust gas concentration to a level lower than an lower explosion limit (LEL) and mix the diluted exhaust gas with an extremely large amount of air before the exhaust gas is discharged into the atmosphere.
With the recent increased awareness with respect to the environmental protection, severer administrative measures for the environmental protection have been taken to strictly regulate the discharge of such exhaust gas to the atmosphere. Accordingly, there is a need to positively remove harmful substances of the exhaust gas discharged from a semiconductor manufacturing process.
Examples of various methods proposed so far for positively removing harmful substances from the exhaust gas include:
(1) scrubbing the exhaust gas by a scrubber method using a basic or acidic agent (in general, an alkaline agent such as caustic soda); PA1 (2) removing harmful component gases by allowing the exhaust gas to pass through layers of adsorbents capable of chemically combining with the harmful component gases; PA1 (3) employing a dry thermal oxidation method in which the exhaust gas is introduced into a flame atmosphere charged with a large amount of hydrogen gas to remove harmful substances; and PA1 (4) employing another dry thermal oxidation method in which the exhaust gas together with oxygen gas (air), nitrogen gas or a like gas is introduced concentrically into a cylindrical electric column heater to decompose harmful substances.
These methods, though having some advantages, involve the following problems, and are required to be further improved by users.
The method (1) is suitable for the treatment of an exhaust gas at a high discharge rate. However, the application thereof is limited to hydrolyzable exhaust gases, and no effect is enjoyed against non-hydrolyzable exhaust gases. More specifically, it is extremely difficult to reduce the concentration of exhaust gas containing non-hydrolyzable gases to a level lower than the threshold limit value (TLV) determined by international standards. Therefore, it is a current practice to mix the treated exhaust gas having a concentration higher than the TLV with an extremely large amount of air before the discharge thereof into the atmosphere.
In accordance with the method (2), exhaust gas is introduced into an adsorption column. The adsorption column has a limited adsorption capacity, thereby requiring column replacement when the adsorption capability thereof is lost. The column replacement entails dangerous operations and disposal of absorbent, resulting in an increased running cost.
In accordance with the method (3), exhaust gas is oxidized in a hydrogen flame atmosphere. Though the effectiveness and treatment capacity of this method have been appreciated, this method involves a safety problem concerning the handling of the hydrogen flame in a semiconductor manufacturing plant. To provide an appropriate safeguard thereagainst, the running cost is increased.
The method (4) which employs a cylindrical column heater suffers from limitations in the heat transfer to exhaust gas and the treatment capacity. Therefore, it is difficult to satisfy various requirements for a semiconductor manufacturing plant. Further, this method involves practical problems such that a hydrolyzable component, acidic component or basic component of the exhaust gas cannot be effectively removed, and dust generated in the exhaust gas cannot be sufficiently removed over a long time.
It is, therefore, a primary object of the present invention to provide a method and apparatus capable of safely and assuredly removing harmful substances of exhaust gas discharged from a semiconductor manufacturing process, which requires a reduced running cost and ensures a stable operation during a prolonged continuous use.
It is another object of the present invention to provide method and apparatus capable of reducing the concentration of any kind of harmful component of exhaust gas to a level lower than the TLV thereof to meet various requirements of semiconductor manufacturing.
It is still another object of the present invention to provide a compact apparatus for removing harmful substances of exhaust gas, which can be used in side-by-side relation to a semiconductor production apparatus in a clean room.